1. Field of the Invention
The present invention relates to methods for testing a connection between speakers and a power amplifier which detect whether or not speakers are appropriately connected to the power amplifier for an audio device mounted on a car, and devices which implement these methods.
2. Description of the Related Art
Car audio devices are installed in almost all cars and are indispensable to the comfort of car users. Audio devices may be installed in a car on a production line during assembly of the car, or a user may purchase their favorite audio devices and ask car dealers and car-related equipment distributors to install the audio devices.
With such a car audio device, speakers are fixed at positions which provide a good acoustic effect in a car and which do not disturb the user in the car. The speakers are connected to a head unit that is attached to, for example, a front panel of a car for use. In many cases, this head unit has a radio receiving function and, in addition, has built-in devices such as a CD player, a television receiver and a DVD player, and further has a navigation function and, in some cases, is connected with a disk exchanger for utilizing CDs or DVDs.
This head unit amplifies the sound volume by means of an amplifier to output audio sounds of these devices or sounds (hereinafter abbreviated as “sounds”) from speakers with an adequate sound volume. In this case, the sound is outputted from the speakers having the positions and sound quality adjusted to provide a good acoustic effect as much as possible in a car or exhibit an acoustic effect of, for example, the 4.1 surround.
Hence, the head unit has a power amplifier to output the sound with the speakers at a sound volume that the user desires. The speakers are provided at various positions in a car and may be connected to the amplifier, where each speaker is connected to the amplifier with a connection cord connected to a connection terminal in the amplifier. By this means, the sound amplified by the amplifier with an adequate balance per speaker is outputted with a predetermined sound quality, with the sound volume the user desires as a whole in a car and with an adequate balance of the sound volume per speaker.
In such a car audio device, appropriately connecting a power amplifier and car speakers is important. To appropriately connect a power amplifier and car spearks, each speaker needs to be appropriately connected with respective connection terminals set in advance with connection cords. Additionally, while connecting the car speakers it is important to avoid directly contacting a conductive material in the car or contacting a power source system such as a battery, causing a short-circuit.
Hence, a product line for cars on which audio devices are installed generally includes a testing procedure of checking whether or not the audio devices are appropriately connected within the cars after the devices are installed in the cars. Further, even when a user additionally purchases a favorite audio device and asks a car dealer or a car-related device distributor to attach this audio device as described above, a worker needs to finally test whether or not the audio device is appropriately attached. Furthermore, even when a wiring system of speakers is repaired after the wiring system is damaged due to, for example, an accident during use of a car, it is necessary to test connections between a power amplifier and the speakers.
Various methods have been conventionally used to test connections between speakers and a power amplifier. A conventional example 1 is illustrated in FIG. 4(a). In this example, a mode of a speaker connection test with respect to a power amplifier illustrated in FIG. 4(a) shows a mode of a speaker connection test with respect to a power amplifier which amplifies an audio signal using a battery voltage of a car, as illustrated in FIG. 4(a-1), and presents an amplified waveform as illustrated in FIG. 4(a-2).
As illustrated in FIG. 4(a-2), when an audio signal is amplified using the battery voltage, a waveform having 6 V of a midpoint potential in the center with respect to a battery voltage of 12 V, which a car provides, is formed with respect to this battery voltage. In addition, an actual waveform in this case is formed as illustrated as a whole in a digital amplifier according to a digital signal in many cases.
With a mode of testing short-circuiting upon testing of the power amplifier in FIG. 4(a-2), the power amplifier which operates as described above measures the potential which is provided during the operation of this power amplifier, at a portion of a speaker connection terminal. The power amplifier determines that short-circuiting occurs on the battery potential side when a voltage higher than 6 V of the midpoint potential is detected and determines that short-circuiting occurs on a ground potential side when a voltage lower than the midpoint potential is detected.
In addition, although, upon this determination, it is theoreticaly possible to determine whether or not short-circuiting occurs by detecting whether or not, for example, the potential is 12 V or 0 V, the battery potential does not maintain 12 V at all times. Various decreases in the voltage may occur and there are cases where the ground potential is not 0 V at all times, and therefore on which side short-circuiting occurs is determined by determining whether the potential is greater or smaller than the midpoint potential as described above.
By contrast with this, a car audio device plays back music while a car is running in an environment of loud noise caused by sources such as an engine sound of a car, a driving sound, wind noise or the sound of cars running side by side. Hence, the car audio device requires a larger sound volume than a household audio device, and the car audio device may have difficulty in meeting a user's demand to output sounds at a high volume with adequate sound quality utilizing 12 V of voltage from a battery installed in a common passenger car. Therefore, the audio device mounted on a car boosts 12 V of battery power source to, for example, 30 V or 40 V to obtain desired power to operate the power amplifier.
Japanese Patent Application Laid-Open No. 2007-74119 discloses a technique of invalidating a detection result of a current detecting unit by means of a voltage detecting unit when a large current flows in speakers and preventing error detection of short-circuiting by detecting that a power source current exceeds an upper limit current value and outputting a detection signal at a high level. The technique includes forcibly switching the detection signal to a low level when detecting that the output voltage exceeds the upper limit voltage value, and enabling both of accurate detection and prevention of error detection of short-circuiting in an amplifier by validating a detection result of the current detecting unit by means of the voltage detection unit upon actual short-circuiting and outputting the detection signal at a high level.
Japanese Patent Application Laid-Open No. 2010-34775 discloses a technique of accurately detecting output short-circuiting in an amplifier with a simple circuit configuration. In the technique two comparison pulse signals are generated based on each signal level of two output stage input pulse signals generated based on an input pulse signal and inputted to an output stage of the amplifier, and a predetermined generation threshold, and outputting a short-circuiting detection signal according to a level comparison result obtained by comparing the signal level of an output pulse from this amplifier and a predetermined detection threshold.
Furthermore, Japanese Patent Application Laid-Open No. 2000-244256 discloses a technique of controlling the bias of a bias circuit to rapidly rise by detecting ground fault or supply fault in a voltage detection circuit based on a midpoint voltage of two output transistors when a power source is activated in a state of ground fault or supply fault. Destruction of the output transistors is prevented due to supply fault or ground fault upon activation of the power source by quickly placing a driving stage transistor and a ground fault/supply fault detection circuit in steady states.
Still further, Japanese Patent Application Laid-Open No. 2006-60278 discloses a technique of reliably detecting short-circuiting of speaker outputs and preventing destruction of an amplifier due to power source pumping. The technique includes providing a direct current voltage detection circuit which detects a direct current voltage produced in speaker output terminals, a control circuit which performs a protection operation when the direct current voltage detection circuit outputs a detection signal and a midpoint potential detection circuit which detects a difference between the midpoint potentials of a positive power source voltage +V and a negative power source voltage −V, connecting the output of the midpoint potential detection circuit to the input of the direct current voltage detection circuit and having the direction current voltage detection circuit output the detection signal when the difference between the midpoint potentials is detected.
The power amplifier having the above booster power source uses a battery power source, which is illustrated in FIG. 4(b-1) for example, and is the same as in FIG. 4(a-1), and uses a booster power source which boosts the battery power source to 40 V, as illustrated in FIG. 4(b-2), to amplify and output the waveform illustrated in FIG. 4(a) as a waveform having 20 V of a midpoint potential as illustrated in FIG. 4(b-3).
When a power amplifier having this performance is used, a device having functional blocks as illustrated in, for example, FIG. 5 to conduct the above connection test of speakers is utilized. With a conventional example of a speaker connection test with respect to a power amplifier using a booster power source illustrated in FIG. 5, when a 12 V battery 32 mounted on a car supplies power to a power amplifier 31, a booster power source 33 boosts the power to 40 V with the illustrated example. A power amplifier IC 34 amplifies an audio signal as illustrated in FIG. 4(b-3) to have 20 V of a midpoint potential, and outputs the audio signal from a speaker connection terminal 35 of the power amplifier 31 to a speaker 36 through a wiring connected with the speaker connection terminal 35.
The above speaker connection test with respect to the power amplifier 31 is conducted by a testing device having an additional speaker connection test processing unit or a speaker connection test processing unit built in advance in a power amplifier. The example illustrated in FIG. 5 shows an example where a speaker connection test processing unit 37 provided in advance in the power amplifier 31 conducts the above speaker connection test.
With the example of a speaker connection test processing unit 41, illustrated in FIG. 5, in a state where the power amplifier is powered on, a test signal is adequately inputted and a midpoint potential of the power amplifier IC 34 is acquired according to a command of a midpoint potential acquiring unit 43 from a bidirectional communication unit 42 of a speaker connection test processing unit 41.
A connection test determining unit 45 of the speaker connection test processing unit 41 detects the voltage at the portion of the connection terminal 35 in the power amplifier IC 34 through the bidirectional communication unit 42, and conducts, if necessary, various connection tests as to whether or not speakers are connected, whether the connected speakers short-circuit on a battery power source side or on a ground potential side, whether or not speakers which need to be originally connected are connected, and whether or not there is a wiring mistake in the power amplifier.
In relation to the present disclosure, an illustrated example of the connection test determining unit 45 shows an example where a battery side short-circuiting 46 determining unit determines whether or not short-circuiting occurs on the battery power source side and a ground side short-circuiting 47 determining unit determines whether or not short-circuiting occurs on the ground side. The battery side short-circuiting 46 detection unit outputs the test results from a test result outputting unit 44 to, for example, a display unit.
This connection test determining unit 45 conducts a test according to a mode of a short-circuiting test as illustrated in FIG. 4(b-3) in a mode of a speaker connection test with respect to a power amplifier having the 40 V booster power source illustrated in FIG. 4(b). That is, similar to a mode of a short-circuiting test in FIG. 4(a-2) in the speaker connection test of the power amplifier without the booster power source in FIG. 4(a), whether or not the midpoint potential short-circuits on the battery power side or short-circuits on the ground side is determined.
That is, the connection test determining unit 45 in the speaker connection test processing unit 41 in FIG. 5 performs processing of pulling in the midpoint potential of a signal which is acquired in the midpoint potential acquiring unit 43 and which is currently outputted from the power amplifier, detecting the voltage at the portion of the output terminal of the power amplifier IC 34 and determining whether the midpoint potential short-circuits on the battery potential side or ground side based on whether the portion includes the battery potential or ground potential.
Hence, as illustrated in FIG. 4(b-3), even though 20 V of a midpoint potential needs to be detected when short-circuiting occurs on the ground side, a potential lower than the midpoint potential is detected and therefore it is naturally determined that short-circuiting occurs on the ground side, and, also in cases where short-circuiting occurs on the battery side, a voltage lower than 20 V of the midpoint potential is detected, and therefore it is determined that short-circuiting occurs on the ground side. This is as described in the lower part of FIG. 4 as the problem of the conventional techniques.